An electrostatic precipitator generally comprises a housing having an inlet side and a discharge side and can be used in association with means, e.g. a blower, for inducing a flow of gas through the housing. Within the housing there are provided arrays of collector electrodes in mutually spaced parallel relationship and corona-discharge electrodes. A high-voltage direct-current field is applied between the corona discharge electrodes and the collecting electrodes so that ionization occurs at the discharge electrodes, thereby electrostatically charging the dust particles which are attracted to and collected upon the collecting electrodes.
Such electrostatic precipitators have been found to be highly efficient in the removal of dusts from a gas stream and are effective even for extremely fine dusts and for the handling of large volumes of dust.
However, a problem arises when the dust entrained in the gas stream has a relatively high resistance. In this case, as the dust collects upon the collecting electrode, a reverse-ionization phenomenon is noted and eventually the surface of the collected dust assumes a charge which may be opposite the charge at the surface of the collecting electrode thereby repelling, rather than collecting, additional dust.
As a result, the efficiency of the electrostatic precipitator for collecting high-resistance dust is substantially lowered as reverse ionization takes place or as the tendency toward reverse ionization increases.
It is known to overcome this problem by a chemical-regulating technique which can involve spraying or otherwise dispersing a liquid or solid into the high-resistance dust, the substance having the characteristic of increasing the conductivity of the dust. This conductivity-promoting substance may be a low-resistance material which, in association with the high-resistance dust, reduces the overall resistance of the substances attracted to the collecting electrodes.
When a liquid is used, it may contain sulfur dioxide, ammonia, triethyl amine, etc. The low-resistance dust may be any waste material having a substantially lower resistance than the high-resistance dust to be recovered from the gas stream.
Since the users of an electrostatic precipitator as well as individuals in the neighborhood thereof strongly object to the discharge of such toxic, noxious or polluting chemical liquids from a flue or stack fed from the electrostatic precipitator, it has been impossible to add even traces of such liquids without violating human sensibilities or environmental control laws.
In practice it is found that the addition of low-resistance dusts or powders to the gas stream entering the electrostatic precipitator is also undesirable because it increases the loading of the electrostatic precipitator and frequently appears to the user thereof to be irrational even if the admixed quantity of dust is fully recovered.